1. Field of the Invention
The present invention relates to an adjusting method of image reading apparatus and an image reading apparatus, in particular, those suitable for use in and as an image reading apparatus, such as an image scanner, a copying machine, and a facsimile apparatus, in which image information of an original is imaged on the surface of a reading means (CCD), making it possible to perform image reading with high accuracy.
2. Related Background Art
FIG. 8 is a schematic view of a main portion of a conventional carriage-integrated type image reading apparatus (flat head scanner).
In the drawing, reference numeral 82 indicates an original table glass, on which an original 81 is placed. Reference numeral 88 indicates a carriage, which retains an illumination light source 83 described below, a plurality of reflecting mirrors 85a, 85b, 85c, and 85d, an imaging lens 86, a reading means 87, etc., and performs scanning in the sub scanning direction in the drawing by means of a sub scanning mechanism 89, such as a motor, thereby two-dimensionally reading the image information of the original 81. The image information is sent to a personal computer or the like, which is an external apparatus, through an interface (not shown).
The illumination light source 83 consists of a xenon tube, a halogen lamp, an LED array, or the like. Inside the carriage 88, the reflecting mirrors 85a, 85b, 85c, and 85d bend light beam from the original 81. The imaging lens 86 forms an image of the light from the original 81 on the surface of the reading means 87. The reading means 87 consists of a linear sensor, such as a CCD (charge coupled device), and is composed of a plurality of light receiving elements arranged in the main scanning direction, which is perpendicular to the plane of the drawing.
In the above configuration, to attain a reduction in the size of the carriage-integrated type image reading apparatus (flat bed scanner), it is necessary to reduce the size of the carriage 88. A reduction in the size of the carriage 88 can be achieved by, for example, increasing the number of reflecting mirrors or securing the requisite optical path by effecting reflection a plurality of number of times with a single reflecting mirror.
However, in these methods, the interior construction of the carriage 88 becomes rather complicated, which involves an exact requirement in terms of assembly accuracy and a substantial increase in cost. Further, in proportion to the surface precision of the reflecting mirrors and the number of times that reflection is effected, the imaging performance deteriorates, which also affects the read image.
In view of this, the present applicant has previously proposed, in U.S. Pat. No. 6,507,444 (Japanese Patent Application Laid-Open No. 2000-171705), an image reading apparatus in which there is introduced into the imaging lens an anamorphic lens at least one surface of which is rotationally asymmetric with respect to the optical axis, thereby making the view angle of the imaging lens 88 wider and reducing the subject-to-image distance to shorten the optical path length itself.
Further, an image reading apparatus is known which uses an off-axial reflection surface of a rotationally asymmetrical configuration as a means for solving the problems accompanying widening of view angles, such as generation of chromatic aberration (see, for example, USAA 2003038228 (Japanese Patent Application Laid-Open No. 2002-335375)).
Further, there has conventionally been known an image reading apparatus which obtains information on dirt and flaws on an original surface by using infrared light and performs correction on the corresponding portion in visible image information (see, for example, USAA 2003132384 (Japanese Patent Application Laid-Open No. 2000-324303) and U.S. Pat. No. 6,493,061 (Japanese Patent Application Laid-Open No. 2001-189833)).
FIG. 11 is a schematic diagram showing a main portion of a carriage-integrated type image reading apparatus (flat bed scanner) as proposed in USAA 2003132384 (Japanese Patent Application Laid-Open No. 2000-324303) and U.S. Pat. No. 6,493,061 (Japanese Patent Application Laid-Open No. 2001-189833).
The image reading apparatus as shown in the drawing has as light source means an infrared light source 111 and a visible light source 112. In the drawing, image information on a film 113 constituting the transmissive original to be illuminated by one of the light sources for reading, is transmitted by way of a plurality of turn-back mirrors 116 and an imaging lens 117 to effect imaging on the surface of a CCD (line CCD) 118 in which a plurality of pixels are arranged in the main scanning direction.
The imaging lens 117 has a configuration in which a plurality of glass or resin lenses are contained in a lens barrel. The film 113 is placed on an original table glass 114. A focusing correction element 119 to be described later is disposed between the imaging lens 117 and the CCD 118. A carriage 120 is moved in the sub scanning direction by a driving device 115, and two-dimensional image information of the film 113 is read by the CCD 118.
In this type of image reading apparatus, the imaging position on the surface of the CCD 118 varies due to a difference in wavelength between the infrared light emitted from the infrared light source 111 and the visible light emitted from the visible light source 112, so that the focusing position is adjusted by making the focusing correction element 119 detachable (retractable) with respect to the optical path. Here, the focusing correction element 119 consists of a plane-parallel glass. Due to this configuration, it is possible to read the image information of both infrared and visible images in a satisfactory imaging condition.
The optical system shown in the above U.S. Pat. No. 6,507,444 (Japanese Patent Application Laid-Open No. 2000-171705) and USAA 2003038228 (Japanese Patent Application Laid-Open No. 2002-335375) has an optical element with a refraction surface or reflection surface that is rotationally asymmetrical, so that the imaging area on the CCD surface is also a rotationally asymmetrical area. In particular, as shown in FIG. 9, in an optical system attaining wider view angles, high priority is given to the imaging performance of the main scanning area, which is large, so that the area of high resolving power is large in the main scanning area, whereas it is small in the sub scanning area.
Further, in a carriage-integrated type image reading apparatus, it is rather difficult to maintain high positional accuracy for the parts due, for example, to the large number of components, the great variation in molding accuracy of the carriage in resin molding, which is adopted in view of material cost, etc.
All in all, in a carriage-integrated type image reading apparatus, introduction of an imaging lens (imaging means) with a small imaging area in the sub scanning direction leads to the following problem.
For example, in the carriage-integrated type image reading apparatus shown in FIG. 10, when the parts are accurately positioned, the reading optical path takes the route as indicated by the alternate long and short dashed line a, and reaches (effects imaging on) the surface of the CCD 107. However, when, for example, the reflecting mirror 105c is positionally deviated, the reading optical path takes the route as indicated by the chain double-dashed line b from the reflecting mirror surface before reaching a position in the vicinity of the CCD 107.
In view of this, in the conventional image reading apparatus, in order to obtain satisfactory image information, the CCD 107 is shifted for adjustment in the sub scanning direction when receiving image information from the original.
However, in an image reading apparatus having an imaging lens with a rotationally asymmetrical refraction or reflection surface, the imaging area in the sub scanning direction is determined according to the position of the imaging lens 106, so that performing shift adjustment as mentioned above results in the CCD 107 being deviated from the imaging area in the sub scanning direction, making it impossible to obtain satisfactory image information.
In the carriage-integrated type image reading apparatus shown in FIG. 11, it is possible to read both visible image information and infrared image information in a satisfactory imaging condition. However, the apparatus involves the following problems.
(Problem 1: Size of the Focusing Correcting Means)
Light beam with image information increases in width in the main scanning direction as it is departed from the imaging lens 117. Thus, in the conventional image reading apparatus, in which the focusing correction element 119 is disposed at a position spaced apart from the imaging lens 117, it is rather difficult to attain a reduction in the size of the focusing correction element 119. When the focusing correction element 119 is large, it is necessary for the driving device, such as a motor or an electromagnet, for moving the focusing correction element 119 to be relatively powerful and large.
(Problem 2: Problem in Assembly Process)
For example, in an image reading apparatus using an ultra-wide angle lens as disclosed in U.S. Pat. No. 6,507,444 (Japanese Patent Application Laid-Open No. 2000-171705), the optical path length is small, so that the distance between the imaging lens 117 and the CCD 118 is small, and it is rather difficult to incorporate the focusing correction element 119, with the result that the assembly process is complicated, resulting in a deterioration in process yield and an increase in assembly time.
(Problem 3: Dirt and Flaws on the Imaging Lens)
In addition to Problem 2, when incorporating the focusing correction element 119 into the small space, the imaging lens 117, the CCD 118, etc. may be inadvertently touched, resulting in dirt such as a finger mark, flaws, etc. being imparted thereto. In particular, with a lens formed by using a cycloolefin polymer type resin or the like, it is impossible to wash away any dirt thereon.